Due to fast and frequently switch between pumping and generating modes several times daily and continuously increase to operate at off-design conditions, understanding the detail of flows is a prerequisite to the design and safety operation of a pump-turbine. Because complex flow phenomena occur inside the draft tube at different operating conditions, its flow patterns are highly demanding to be accurately investigated with reliable approach. Laser Doppler Velocimetry measurements and vortex ropes visualization inside the draft tube of a high-head pump-turbine with the specific speed n(q) = 24 have been performed at various operating conditions both in turbine and pump modes. Measurements and observations were taken for the speed factor n(ED) = 0.21 and 0.19 at loads ranging from 40% to 120% of the rated power at turbine mode, and the possible operating range at pump mode. The distribution of measured axial and tangential velocities, the size of backflow zones and the rotation of vortex rope are all in line with the hydraulic design and model test results. The LDV measurement has been applied to accurately analyze the swirl flow pattern and quantify swirling intensity, the region of backflow and vortex rope, as well as to precisely determine the location of the rope-free zone combining with vortex ropes visualization. Measured instantaneous tangential velocities have been applied to accurately distinguish the processional frequency of a rotating vortex rope inside the draft tube from other fluctuations at part loads. Influence of the splitter blades can be also observed in both time and frequency domain of measured instantaneous velocities. This study can provide rather valuable information for analysis and validation in different operating regimes. The presented methodology and techniques have been demonstrated very helpful to successfully developed high-head pump-turbines for two pumped storage power plants in China. (C) 2018 Elsevier Ltd. All rights reserved.